W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 systems (International Mobile Telecommunication system), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4, Release 5 and Release 6.
A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
However, knowing that user and operator requirements and expectations will continue to evolve, the 3GPP has begun considering the next major step or evolution of the 3G standard to ensure the long-term competitiveness of 3G.
The 3GPP recently launched a study item “Evolved UTRA and UTRAN” better known as “Long Term Evolution (LTE)”. The study will investigate means of achieving major leaps in performance in order to improve service provisioning, and to reduce user and operator costs. It is generally assumed that Internet Protocols (IP) will be used in mobility control, and that all future services will be IP-based. Therefore, the focus of the evolution is on enhancements to the packet-switched (PS) domain of legacy UMTS systems.
The main objectives of the evolution are to further improve service provisioning, and reduce user and operator costs, as already mentioned. More specifically, some key performance, capability and deployment requirements for the long-term evolution (LTE) are inter alia:                significantly higher data rates compared to HSDPA and HSUPA (envisioned are target peak data rates of more than 100 Mbps over the downlink and 50 Mbps over the uplink),        Mean user throughput improved by factors 2 and 3 for respectively uplink (UL) and downlink (DL),        high data rates with wide-area coverage,        cell-edge user throughput improved by a factor 2 for uplink and downlink,        uplink and downlink spectrum efficiency respectively improved by factors 2 and 3,        significantly reduced latency in the user plane in the interest of improving the performance of higher layer protocols (for example, TCP) as well as reducing the delay associated with control plane procedures (for instance, session setup), and        stand-alone system operation in spectrum allocations of different sizes ranging from 1.25 MHz to 20 MHz.        
One other deployment-related requirement for the long-term evolution study is to allow for a smooth migration to these technologies.
The ability to provide high bit rates is a key measure for LTE. Multiple parallel data stream transmission to a single terminal, using multiple-input-multiple-output (MIMO) techniques, is one important component to reach this. Larger transmission bandwidth and at the same time flexible spectrum allocation are other pieces to consider when deciding what radio access technique to use. The choice of adaptive multi-layer OFDM (Orthogonal Frequency Division Multiplexing), Adaptive Multi Layer (AML)-OFDM, in downlink will not only facilitate to operate at different bandwidths in general but also large bandwidths for high data rates in particular. Varying spectrum allocations, ranging from 1.25 MHz to 20 MHz, are supported by allocating corresponding numbers of AML-OFDM subcarriers. Operation in both paired and unpaired spectrum is possible as both time-division and frequency-division duplex is supported by AML-OFDM.
OFDM with Frequency-Domain Adaptation
The AML-OFDM-based downlink has a frequency structure based on a large number of individual sub-carriers with a spacing of 15 kHz. This frequency granularity facilitates to implement dual-mode UTRA/E-UTRA terminals. The ability to reach high bit rates is highly dependent on short delays in the system and a prerequisite for this is short sub-frame duration. Consequently, the LTE sub-frame duration is set as short as 1 ms in order to minimize the radio-interface latency. In order to handle different delay spreads and corresponding cell sizes with a modest overhead, the OFDM cyclic prefix length can assume two different values. The shorter 4.7 ms cyclic prefix is enough to handle the delay spread for most unicast scenarios. With the longer cyclic prefix of 16.7 ms very large cells, up to and exceeding 120 km cell radiuses, with large amounts of time dispersion can be handled. In this case, the length is extended by reducing the number of OFDM symbols in a sub-frame.
The basic principle of Orthogonal Frequency Division Multiplexing (OFDM) is to split the frequency band into a number of narrowband channels. Therefore, OFDM allows transmitting data on relatively flat parallel channels (subcarriers) even if the channel of the whole frequency band is frequency selective due to a multipath environment. Since the subcarriers experience different channel states, the capacities of the subcarriers may vary and permit a transmission on each subcarrier with a distinct data-rate. Hence, subcarrier wise (frequency domain) Link Adaptation (LA) by means of Adaptive Modulation and Coding (AMC) increases the radio efficiency by transmitting different data-rates over the subcarriers.
OFDMA allows multiple users to transmit simultaneously on the different subcarriers per OFDM symbol. Since the probability that all users experience a deep fade in a particular subcarrier is very low, it can be assured that subcarriers are assigned to the users who see good channel gains on the corresponding sub-carriers. When allocating resources in the downlink to different users in a cell, the scheduler takes information on the channel status experienced by the users for the subcarriers into account. The control information signaled by the users, i.e. CQI, allows the scheduler to exploit the multi-user diversity, thereby increasing the spectral efficiency.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of evolved Node Bs (eNB), providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the mobile node (referred to in the following as UE or MN).
The eNB hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. Further, it performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL-QoS (Quality of Service), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME, and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs.
The S-GW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and Packet Data Network Gateway). For idle state UEs, the S-GW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the S-GW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the Home Subscriber Server, HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.
Hybrid ARQ Schemes
A common technique for error detection and correction in packet transmission systems over unreliable channels is called Hybrid Automatic Repeat reQuest (HARQ). Hybrid ARQ is a combination of Forward Error Correction (FEC) and ARQ.
Automatic Repeat-reQuest (ARQ) is an error control method for data transmission which uses acknowledgments and timeouts to achieve reliable data transmissions. An acknowledgment is a message sent by the receiver to the transmitter to indicate that it has correctly received a data packet. If the sender does not receive an acknowledgment before the timeout, being a reasonable time interval for receiving an acknowledgment, it usually re-transmits the frame until it receives an acknowledgment or exceeds a predefined number of re-transmissions.
Forward error correction is employed to control errors in data transmissions, wherein the sender adds redundant data to its messages. This enables the receiver to detect whether an error has occurred, and further allows to correct some errors without requesting additional data from the sender. Consequently, since within a certain limit FEC allows to correct some of the errors, re-transmission of data packets can often be avoided. However, due to the additional data that is appended to each data packet, this comes at the cost of higher bandwidth requirements.
If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly (errors are usually checked by a CRC (Cyclic Redundancy Check)), the receiver requests a re-transmission of the packet.
Depending on the information (generally code bits/symbols) of which the transmission is composed of, and depending on how the receiver processes the information, the following hybrid ARQ schemes are defined:    Type I: The error detection information (such as CRC) is added to the data packet, which is then encoded with a forward error correction code (such as Reed-Solomon code or Turbo code). In the receiver the FEC code is decoded and the quality of the packet is determined. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can decode the actual data packet correctly. If the channel quality is bad and not all transmission errors can be corrected, then the received coded data packet is discarded and a re-transmission for said data packet is requested by the receiver. In this type of HARQ the re-transmission uses the same FEC code as during the initial transmission. Further, the re-transmission data packets contain identical information (code bits/symbols) to the initial transmission. Resulting from the above, the received transmissions are all decoded separately in the receiver.    Type II: According to the second type of HARQ, if the receiver fails to decode a packet correctly, the receiver stores the information of the (erroneously received) encoded packet as soft information (soft bits/symbols) and a re-transmission is requested from the sender. This implies that a soft-buffer is required at the receiver. Re-transmissions can be composed out of identical, partly-identical or non-identical information (code bits/symbols) compared to the initially transmitted data packet. When receiving a re-transmission, the receiver combines the stored information from the soft buffer and the currently received information and tries to decode the packet based on the combined information. The combining of transmissions refers to so called soft-combining, where multiple received code bits/symbols are likelihood combined and solely received code bits/symbols are code combined. Common methods for soft-combining are Maximum Ratio Combining (MRC) of received modulation symbols and log-likelihood ratio (LLR) combining (LLR combining only works for code bits). Type II HARQ schemes are more sophisticated than Type I HARQ schemes, since the probability for a correct reception of a data packet increases with each received re-transmission. This increase comes at the cost of a HARQ soft-buffer at the receiver.            The Type II HARQ scheme can be used to perform dynamic link adaptation by controlling the amount of information to be re-transmitted. E.g. if the receiver detects that decoding has been almost successful, it can request only a small piece of information for the next re-transmission (smaller number of code bits/symbols than in the previous transmission) to be transmitted. In this case it might happen that it is even theoretically not possible to correctly decode the re-transmission packet by itself, wherein this is referred to as non-self-decodable re-transmissions.            Type III: This is a subset of the Type II HARQ with the restriction that each transmission, be it an initial or a re-transmission, must be self decodable.
The HARQ mechanism has been defined for unicast data transmissions, wherein there are two levels of re-transmissions for providing reliability, namely, the Hybrid Automatic Repeat reQuest (HARQ) at the MAC layer and the outer ARQ at the RLC layer.
L1/2 Control Signaling
In order to provide sufficient side information to correctly receive or transmit data in systems employing packet scheduling, so-called L1/L2 control signaling needs to be transmitted. The scheduled users need to be informed about their allocation status, transport format and other data related information (e.g. Hybrid ARQ (Automatic Repeat-reQuest)), using the L1/L2 control signaling transmitted on the downlink along with the data. The control signaling needs to be multiplexed with the downlink data in a sub frame (assuming that the user allocation can change from sub frame to sub frame). Here, it should be noted, that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling then needs only to be transmitted once per TTI. However, in some cases it may make sense to repeat the L1/2 control signaling within a TTI in order to increase reliability.
Typical operation mechanisms for downlink and uplink data transmission when employing ARQ schemes are detailed below:                Downlink Data Transmission:                    Along with the downlink packet data transmission, L1/L2 control signaling is transmitted on a separate physical channel. This L1/L2 control signaling typically contains information on:                            The physical resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.                The transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. In some cases the modulation scheme maybe signaled explicitly.                Hybrid ARQ (HARQ) information:                                    Process number: Allows the UE to identify the hybrid ARQ process on which the data is mapped                    Sequence number or new data indicator (NDI): Allows the UE to identify if the transmission is a new packet or a retransmitted packet                    Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation)                                                UE Identity (UE ID): Tells for which UE the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.                                                Uplink Data Transmission:                    To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink to tell the UE about the transmission details. This L1/L2 control signaling typically contains information on:                            The physical resource(s) on which the UE should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).                The transport Format, the UE should use the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.                Hybrid ARQ information:                                    Process number: Tells the UE from which hybrid ARQ process it should pick the data                    Sequence number or new data indicator: Tells the UE to transmit a new packet or to retransmit a packet                    Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation)                                                UE Identity (UE ID): Tells which UE should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.                                                
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. E.g.:                HARQ process number may not be needed in case of a synchronous HARQ protocol        A redundancy and/or constellation version may not be needed if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre defined.        Power control information may be additionally included in the control signaling        MIMO related control information, such as e.g. precoding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included        
In case of uplink data transmission, part or all of the information listed above may be signaled on uplink, instead of on the downlink. E.g., the base station may only define the physical resource(s) on which a given UE shall transmit, then he transport format, modulation scheme and/or HARQ parameters may be selected be the UE and, therefore, signaled on the uplink. Which proportion of this information is signaled on the uplink and which proportion is signaled on the downlink is typically a design issue and depends on the view how much control should be carried out by the network and how much autonomy should be left to the UE.
Generally, the information sent on the L1/L2 control signaling may be separated into the following two categories:                Shared Control Information (SCI) carrying Cat 1 information. The SCI part of the L1/L2 control signaling contains information related to the resource allocation (indication). The SCI typically contains the following information:                    User identity, indicating the user which is allocated.            RB allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can be dynamic.            Optional: Duration of assignment, if an assignment over multiple sub frames (or TTIs) is possible.                         Depending on the setup of other channels and the setup of the Dedicated Control Information (DCI), the SCI may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).        Dedicated Control Information (DCI) carrying Cat 2/3 information. The DCI part of the L1/L2 control signaling contains information related to the transmission format (Cat 2) of the data transmitted to a scheduled user indicated by Cat 1. Moreover, in case of application of (hybrid) ARQ it carries HARQ (Cat 3) information. The DCI needs only to be decoded by the user scheduled according to Cat 1. The DCI typically contains information on:                    Cat 2: Modulation scheme, transport block (payload) size (or coding rate), MIMO related information, etc. (It should be noted that either the transport-block (or payload size) or the code rate can be signalled. In any case, these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated RBs.)            Cat 3: HARQ related information, e.g. hybrid ARQ process number, redundancy version, re-transmission sequence number.Persistent Scheduling                        
In the downlink, E-UTRAN can dynamically allocate resources to UEs at each TTI via the C-RNTI (Cell-Radio Network Temporary Identifier) on the L1/L2 control channel(s). A UE always monitors the L1/L2 control channel(s) in order to find possible allocation when its downlink reception is enabled (activity governed by DRX (discontinuous reception)).
In addition, E-UTRAN can allocate persistent downlink resources for first (initial) transmissions to UEs. When required, re-transmissions are explicitly (dynamically) signalled via the L1/L2 control channel(s) to the UE, since an asynchronous HARQ protocol is used in the downlink. Due to the asynchronous property of the downlink, the UE needs to be signalled explicitly for which data packet the re-transmission is intended In more detail, e.g. an N-process stop-and-wait HARQ protocol may be employed that has asynchronous re-transmissions in the DL and synchronous re-transmissions in the UL. Synchronous HARQ means that the re-transmissions of HARQ blocks occur at pre-defined periodic intervals. Hence, no explicit signaling is required to indicate to the receiver the re-transmission schedule. Conversely, asynchronous HARQ offers the flexibility of scheduling re-transmissions based on air interface conditions. In this case, some identification of the HARQ process needs to be signaled in order to allow for a correct combining and protocol operation.
Furthermore, a UE can be allocated to persistent resources. In the sub-frames where the UE has persistent resource, if the UE cannot find its C-RNTI on the L1/L2 control channel(s), a downlink transmission according to the persistent allocation that the UE has been assigned in the III is assumed.
There are several possibilities to discriminate between L1/L2 control signaling for activation/deactivation of a persistent grant and an allocation for a dynamic grant. Using an additional field for discrimination in the control information, an extra C-RNTI for persistent allocation or a codepoint in an existing field of the control information are among these possibilities.
Moreover, in the sub-frames where the UE has persistent resource, if the UE finds its C-RNTI on the L1/L2 control channel(s), the L1/L2 control channel allocation overrides the persistent allocation for that TTI, and the UE does not decode the persistent resources.
The objective of persistent scheduling is to reduce Layer 1 and Layer 2 control channel overhead, especially for VoIP traffic.
In FIG. 3 there is illustrated how persistently scheduled data packets (protocol data units) are transmitted between an evolved NodeB (eNB) and a mobile node (UE), wherein a PDCCH channel (Physical Downlink Control CHannel) is used to provide control information (L1/L2 control signaling) from the eNB to the UE so that the UE knows when, where and how to receive dynamically scheduled data. As already mentioned, when using persistent scheduling, the resources for transmitting first transmissions (1st tx) of data are allocated persistently (e.g. fixed time slots, e.g. every 20 ms), whereas re-transmissions of data (re-tx) are scheduled dynamically, i.e. depending on the current conditions of the communication channel. FIG. 3 is divided into the upper part, relating to the eNB which transmits data to the UE and into the middle part, relating to the UE which received the data. The lower part of the figure illustrates the control channel via which control information is transmitted from the eNB to the UE.
It is assumed that the eNB transmits three protocol data units to the UE in the downlink, by using one HARQ process (e.g. ID=4) reserved to persistent scheduling, as illustrated in FIG. 4, and with time intervals of e.g. 20 ms. As apparent from FIG. 3, the first protocol data unit is received correctly by the HARQ process in the UE, the HARQ process in turn sending an ACK-message back to the eNB. The subsequent protocol data unit (second PDU) is transmitted 20 ms later than the first, and is not received correctly in the HARQ process. Consequently, the HARQ process in the UE transmits a NACK-message to the eNB for instructing it to perform a re-transmission for the second PDU since it wasn't correctly received. As mentioned previously, the re-transmission might be composed of identical, partly-identical or non-identical information (code bits/symbols) compared to the initially transmitted data packet (PDU) (see Type II of HARQ).
Furthermore, the re-transmissions in the downlink are scheduled dynamically, i.e. control information is transmitted to the UE, so that the UE knows when, where and how to receive the re-transmission of the second protocol data unit. The control information transmitted to the UE over the PDCCH channel comprises amongst other things the HARQ process ID (ID=4) of the HARQ process which is reserved for persistent scheduling. In accordance therewith, the UE receives the re-transmission of the second protocol data unit (re-tx) and stores same in the HARQ process ID=4 in which the initial transmission (1st tx) of the second PDU is already stored. Then, soft-combining is performed with the two received transmissions relating to the second PDU (1st tx and re-tx). In case soft-combining is successful, the second PDU is correctly decoded, and an ACK-message is provided to the eNB.
In case soft-combining is not successful, the second PDU cannot be decoded, and the UE needs to request a further re-transmission from the eNB by sending a NACK-message. However, since the second re-transmission for the second PDU would be effected after the initial transmission of the third PDU, the second re-transmission for the second PDU is not possible. In more detail, when having only one HARQ process reserved for persistent scheduling, each time a new PDU is transmitted to the UE, the corresponding reserved HARQ process, i.e. the associated memory, has to be flushed (emptied) in order to store the initial (and possible re-transmissions) for the new PDU. Thus, there is a time constraint for the re-transmissions for a specific PDU. Said time constraint limits the time interval for possible re-transmissions to the persistent scheduling cycle (in this example, 20 ms). Accordingly, the initial transmission and all re-transmissions for one specific PDU have to be successfully accomplished within the persistent scheduling cycle; otherwise, the specific PDU cannot be decoded correctly, in case the initial transmission and the re-transmission that are received in time cannot be soft-combined successfully to form the specific PDU.
Apparently, depending on the actual persistent scheduling cycle, this time constraint for re-transmissions can be quite stringent, which is disadvantageous for the system performance.
It would be possible to assign more than one HARQ process to the transmission of persistent scheduled data, in order to loosen said time constraint for re-transmissions. However, this in itself also implicates some disadvantages. For instance, currently 8 HARQ process are being intended by the standardization bodies; thus, 25% of all available HARQ processes would be reserved for persistent scheduling, and would thus not be available for the usual dynamic scheduling. Furthermore, the maximum achievable data rate of the UE is lowered as well.
Assuming that a normal dynamic scheduling scheme is utilized instead of the persistent scheduling, similar problems arise.
Furthermore, the overall number of HARQ processes is usually limited to 8, which might be disadvantageous in case several applications are running at the same time using the HARQ protocol.